Do you think the slow shutters speed only worked because he was panning?
Yes, he says this in the video, although he didn't make it terribly clear. Obviously, if you are holding the camera steady, shutter speed will have no effect on how visible the fence is. I have used very slow shutter speeds to remove people in street scenes. As long as they are moving, it works. If they slow down or stop for a significant portion of the exposure, you can see them. If you are tracking a car it is staying relatively still in the frame, but the fence will be "moving" in relation to the camera, and will not be clearly visible depending on speed of movement and shutter speed.
